High performance PE100 Resin with extraordinary resistance to Slow
Crack Growth
Tetsuya Yoshikiyo and Takaaki Hattori, Japan Polyethylene Corp.

Abstract
Polyethylene, especially PE100 Resin has been steadily increasing its use in pressure pipes for waterand gas based on its superior property balance. Of all the properties required for PE100, when we
consider the defects on the pipe surface and the stress concentrations on the fitting of complicatedshape, the most important for the lifetime of pipe should be the resistance to Slow Crack Growth(SCG). Recent requirements for cost reduction by no-dig or no-sand installation have been enhancing
the need for the improvement of SCG resistance. Requirements to Rapid Crack Propagation (RCP)
for higher operating pressure and no sag for large diameter - thick wall pipe are also increasing.
To meet these requirements, we investigated the relationship between molecular architecture andresin properties in detail, and we optimized the molecular weight, molecular weight distribution,
co-monomer, co-monomer distribution using our advanced multi-modal polymerization process andhigh activity catalyst. Our new PE100 resin has the Notch Pipe Test result of more than 17,000 h.
This value is far above the typical values of PE100 resins so far on the market and more than 3 times
greater than the requirement for PEX pipe. It has the RCP-S4 value of more than 25 bar, and its
designed high shear sensitivity also gives a good no sag property without sacrificing
extrusion/injection processing properties.
We believe that continuous improvements of resin performance will help PE pipe to continue toexpand, meeting more demanding new installation methods and contributing to enhance the safetymargin of the PE pipe system.

Introduction
Polyethylene(PE) has already established its position as a major material for many pipe applications,such as gas and water distributions (pressure pipes), sewerage, drainage and conduit, based on it’s
excellent characteristics such as light weight/ flexibility for easy handling and chemical stability forcorrosion resistance. The strength of PE pipe line system to the earthquake according to its ability tofollow the ground movement and excellent fusion-welding strength is now well acknowledged. In
Japan, there have been several strong earthquakes with the magnitude of more than 6 such as theGreat Hanshin-Awaji earthquake (Magnitude 7.3, 1995) and the Niigata-ken Chuetsu earthquake(Magnitude of 6.8, 2004). Although there were a lot of damages observed to iron, steel and PVC
pipes in those earthquakes, there was no report of the damage to the PE gas and water pipe line
system. Therefore, the usage of PE pipes to the lifelines like water and gas is increasing year after
year also in Japan.
PE for pipes, especially PE100 resin, has been steadily increasing its use all over the world and is
expected to grow further. Together with the spreading of PE100 resin, the requirements for costcompetitive installation methods like no-dig or no-sand methods are also increasing. In these
installation methods, however, we can not avoid the surface defect by scratching and theconcentrated local stress by stone or something like that in the backfill material. These defect andstress concentration can give the pipe more stress than anticipated and may cause the failure of thepipe, if the material’s resistance to stress crack or slow crack growth (SCG) is not strong enough.
Electro-fusion (EF) fitting is widespread for pressure pipe systems, because the operation is easy andthe reliability on the fusion-welding strength is high. However, the complicated shape of the fitting
itself and the possible notch shape at the welded part may cause stress concentrations and lead to thefailure, if the design of the fitting is not appropriate and the resistance of the material to SCG is not
sufficient. When we consider these influences that deteriorate the system, the resistance to SCG isextremely important for the life time of the actual pipe line system.
The SCG resistance of the material can be evaluated by several other methods like PENT1 andFNCT2, however, Notch Pipe Test (NPT) according to ISO134793 is used for gas and water
applications (ISO 44374 for gas, ISO 44275 for water) as the most reliable evaluation method
considering the actual usage of the pipe. The value now required for gas and water is more than 165 h.
There are a lot of PE100 resins that meet the criteria of ISO 90806, however, the SCG value of eachPE100 resin varies. Many works have been carried out by many resin manufacturers so far to
improve the SCG properties. And now, some of the PE100 resins have the SCG values far beyond
165 h.
Beside the SCG, the requirements to Rapid Crack Propagation (RCP) for higher operating pressureand no sag property for large diameter - thick wall pipe are also increasing. To meet theserequirements, we investigated the relationship between molecular architecture like molecular weight(MW), molecular weight distribution (MWD), co-monomer, co-monomer distribution and resin
properties in detail, and tried to find the best balance between mechanical properties andprocessability. To achieve the desired molecular architecture, we tried to optimize the polymerization
conditions using our advanced multi-modal polymerization process and high activity catalyst.

Relationship between Molecular Architecture and Polymer Properties
The crack propagation of SCG is considered to happen between crystalline lamellas and the rate of
SCG is considered to relate with the rate of disentanglement of tie-molecules between crystalline
lamellas. So, increasing the number of tie-molecules to delay the disentanglement is considered to be
the key point to improve the SCG. It is generally understood that increasing the co-monomer content
improves the SCG, or, increases the number of tie-molecules. But, it also reduces the stiffness of thematerial. PE100 requires both high stiffness and high durability. The molecular architecture thatrealizes the good property balance is required. Y. L. Huang et al.7 calculated the number oftie-molecules based on the assumption that molecules with larger end to end distance than the criticaldistance (2Lc + La), where Lc means the thickness of the crystal and La means the thickness of theamorphous layer between the crystals, will form a Tie-molecule. Using this theory, we can get theguideline for desired molecular architecture that provides more tie-molecules at the same
co-monomer content. To concentrate the co-monomer in the higher molecular portion and to
distribute the co-monomer evenly within the molecule are two important factors.8
Table 1 shows the summary of the relationship between bi-modal molecular architecture andproperties that are required for PE pipe material. Within these relationships, to find the best balance
and to realize the desired molecular architecture by adjusting the polymerization conditions are our
purpose. Today, bi-modal or multi-modal process that produces high molecular weight co-polymer
and low molecular weight homo-polymer in a series of reactors is usually used. Fig.1 shows thescheme of molecular distribution of a bi-modal PE and the basic design concepts.
Molecular weight
distribution (MWD)

Component ratio
Property

High molecularweight component(HMWC)

Low molecularweight component(LMWC)

Narrow

Broad

+

-

-

+

-

+

+

-

-

+

+

-

Higher creep performance
Higher resistance of
ESC or SCG
Higher tensile strength

+

-

+

-

Higher impact strength+-

Higher stiffness+

Better processability-

Better external appearance-

Lower shrinkage

-

+

-

Note:

+: positive effect- : negative effect

Table 1

Relationship between basic bi-modal molecular architecture and polymer properties.

Low molecular weight component+ higher stiffness+ better processability
High molecular weight component+ higher creep performance+ higher resistance of ESC or SCG+ higher mechanical strengthlog MW

Fig. 1

Scheme of molecular weight distribution of a bimodal PE

Polymerization:

Advanced Multi-modal Slurry Loop Process

Molecular Weight (MW) and Co-monomer Content, which are main factors to affect polymer
properties, are controlled by the polymerization conditions like polymerization temperature,polymerization pressure, residence time, hydrogen partial pressure, co-monomer concentration and soon. In multi-modal process, each condition is controlled independently in each reactor. Precise
control of those factors in each reactor is the key point to obtain the polymer of desired moleculararchitecture. We carried out the development of PE100 material with excellent SCG resistance using
our Advanced Multi-modal Slurry Loop Process (AMSLP) and our high performance Ziegler-Natta
Catalyst.
In multi-modal or cascade reactor process, High Molecular Weight Component (HMWC) and Low
Molecular Weight Component (LMWC) are polymerized in a series, however, the order of thepolymerization is different by manufactures or polymerization processes. In case of thepolymerization that produce the LMWC first, the possibility of poor dispersion of HMWC, Gel orFish Eye, is higher than that of the opposite polymerization order, when we try to increase the MW
difference between the two components to get a good processability and a good stiffness and SCG
balance. So, we used the polymerization order that produce the HMWC first. Not only the MW
difference between the two components but also the ratio of HMWC and LMWC affects thedispersion of HMWC. Fig. 2 shows the bad example of HMWC dispersion. Optimization of MW andratio of each component is very important.

Fig. 2The inner wall of the pipe made from the material which has a poor dispersion of HMWC

As we mentioned above, concentrating the co-monomer in the higher molecular portion anddistributing the co-monomer evenly within the molecule are two important factors in order to get
excellent SCG resistant PE100 material. In other words, catalyst and polymerization conditions that
increase the capability of co-polymerization in the HMWC production, that increase the density of
LMWC as high as possible and that increase the evenness of co-monomer incorporation are essential.
We investigated catalyst, process and polymerization conditions to realize these criteria. Table 2
shows the summary of our investigation. In the table 2, CL/CH means the ratio of co-monomer content
of LMWC to HMWC. In this experiment, we used hexene as co-monomer. We measured the CH
directly using C-13 NMR spectrum9, 10 of HMWC. Regarding CL, we calculated the value using CH
and that value of whole polymer. The lower the value is, the more the co-monomer is concentrated in
the HMWC.
We calculated Tβδ/Tδδ/C as the figure that indicates the evenness of the co-monomer incorporation.
As shown in Fig. 3, Tβδ calculated from the height of 36.0 ppm peak means the number ofsuccessive incorporation of two co-monomers and Tδδ calculated from the height of 38.1 ppm peakmeans the number of the isolated incorporation of co-monomer.11 And C means the co-monomer
content of the polymer. The lower the value is, the more evenly the co-monomer is incorporated.
PE-2 shows the result of polymerization condition optimization to increase the capability of
co-polymerization in the HMWC production and increase the evenness of co-monomer incorporation.
Although the whole polymer’s HLMFR (MW) and density of PE-1 and PE-2 are nearly the same, thevalues of CL/CH and Tβδ/Tδδ/C of the two polymers are different. The lower CL/CH and Tβδ/Tδδ/Cvalues of PE-2 means that PE-2 has more co-monomer in HMWC and has more evenly incorporated
co-monomer than PE-1. As the result of the difference of molecular architectures of the two polymers,
PE-2 has nearly 4 times higher value of Notched Lander ESCR, which we use as a screening test
method for SCG at our laboratory. The details of the test method are shown in Fig. 4.
PE-3 is produced using the three cascade slurry loop reactor process, “Advanced Multi-modal Slurry
Loop Process (AMSLP)”, producing the same specification HMWC in the first and second reactors,
and then LMWC in the third reactor. We produced PE-3 with the same ratio of HMWC to LMWC asthat of PE-1 and PE-2, and adjusted the polymerization condition to get the same HLMFR anddensity as a whole polymer after pelletizing. PE-3 has even lower values of CL/CH and Tβδ/Tδδ/Cthan those of PE-2, and as a result it has even higher value of Notched Lander ESCR compared toPE-2. As these results show, we could confirm that fine control of the molecular architecture by
precise and sophisticated polymerization can further improve SCG resistance.

PE-1

PE-2

PE-3

0.16

0.19

0.16

kg/m

929

927

923

-

HexeneHexeneHexene

%

45

45

23

12

13

0.14

949

949

923

After 1st reactor
HLMFR

g/10min3

Density
Comonomer
Polymerization Percentage at
1st reactor
After 2nd reactor
HLMFR

g/10min3

Densitykg/mComonomer
Polymerization Percentage at
2nd reactor
After 3rd reactor
HLMFR
Density
Polymerization Percentage at
3rd reactor
After palletizing
HLMFR

-

Hexene%55

55

g/10min

23

15

3

kg/m

948

%

54

g/10min
3

9.0

9.9

9.4

Density

kg/m

949

949

948

CL/CH

-

0.40

0.18

0.16

Tβδ/Tδδ/C

-

0.14

0.11

0.09

Notched Lander ESCR

h

70

300

533

Table 2

Typical properties of the investigated materials

a: Tβδ (successive incorporation)b: Tδδ (isolated incorporation)

b

a

PPM
40

Fig. 3

35

30

25

20

15

10

C13-NMR spectrum

Test Conditions-Temperature: 80 degree C-Reagent: 1% alkyl(C12-16) sodium
sulfate solution

Glass cell

-Load: 60kg/cm2-Specimen:

Hot water
at 80 degree C
Razor notch（one side: 0.35mm depth）

Specimen
6mm

16mm

20mm

25mm

20mm

Reagent
Load
Touch Sensor

Fig. 4

Notched Lander ESCR test

Evaluation result of new PE100 material
Based on the results we mentioned in the earlier chapter, we developed new PE100 grade(New-PE100) using our Advanced Multi-modal Slurry Loop Process (AMSLP). In Table 3, wesummarize the evaluation results of this New-PE100 together with those of our previous PE100

grades (PE100-A, PE100-B). In these results, the particularly notable feature is the SCG resistance of
New PE100. We evaluated the SCG resistance by Notched Pipe Test (NPT) according to ISO 13479
using 110mm outside diameter and 10mm thickness pipe, at the test temperature of 80 degree C andinside pressure of 9.2 bar. NPT is now regarded as the most realistic method to evaluate the failure ofsurface scratched pipe.11 This test was performed on 110mm SDR11 pipe using pressure of 9.2 bar at
80 degree C. New-PE100 designed to improve SCG performance did not rupture until 17,200h,
though the performance of our previous PE100 grade (PE100-A) was less than 3,000h. The actual
failure times of the 5 tested specimens were from 17,236 h to 24,186 h. This value is far beyond the
now required value of 165h in gas pipe application (ISO 4437 ) and more than 3 times higher than theminimum required value for the PEX pipe (ISO 14531-1)13 that is often used with the no-dig
installation methods because of it’s superior SCG properties. Fig. 5 shows the stress dependence of
NPT failure time. Compared with PE100-B, New-PE100 has higher hoop stress value at the same
failure time level.
Fig. 6 shows the result of the hydrostatic strength test of New-PE100 according to ISO 9080.
There was no brittle failure at all temperatures during the test period. Fig. 7 shows the S4 (small-scalesteady-state) rapid crack propagation (RCP) performance of New-PE100 according to ISO 1347714
using 110mm diameter and SDR11 pipes. We measured the critical pressure (Pc, S4), at which there isan abrupt transition from arrest to propagation of the initial crack changing the inside pressure level,at 0 degree C. Although we increased the inside pressure until 25 bar, New-PE100 showed no crack
propagation. It shows that New-PE100 has enough RCP strength for higher operating pressure andfor larger diameter-thick wall pipes. This good RCP result is suggested by the excellent Charpy
impact strength of New-PE100.
Property

Unit

MethodNew-PE100

PE100-A

PE100-B

g/10minISO 1133

9

22

9

-

ISO 1133

180

179

180

kg/m

ISO 1183

948

949

950

-

-

HexeneHexeneButeneMPa

ISO 178

1,100

1,100

1,200

kJ/m2

ISO 179

37

22

25

kJ/m2

ISO 179

19

8

10

σLPL

MPa

ISO 9080

Notch pipe testh

ISO 13479

10.22 *
(PE100)> 17,200 *10.45 **
(PE100)2,500 **

10.28 **
(PE100)-RCP-S4

Bar

ISO 13477> 25 *> 16 **-HLMFR
FRR(21.6/2.16)

3

Density
Comonomer
Flexural modulus
Charpy impact strength
at 23 degree C
Charpy impact strength
at -20 degree C* determined at Kiwa Gastec Certification** determined at Bodycote Polymer
Table 3

Evaluation results of New-PE100

6.0
New-PE100
PE100-B

Hoop stress（MPa）

5.5

5.0

4.5

4.0
1

10

100

1000

10000

100000Time to failure（h）
Fig. 5Stress dependence of notched pipe test data

30

Hoop stress（MPa）

20

10

σLPL=10.2

5

20℃
60℃
80℃

LTHS
LPL

1
1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+0550y

1.E+06Time to failure（h）+ There has been no brittle failure during about 11,000h of testing at each temperature.
Fig. 6

Hydrostatic strength data: determined at Kiwa Gastec Certification

110mm-SDR 11 blue pipe

6
5
4

a/dn

PC,S4 > 25 bar
3
2
1
0
0

5

10

15

20

25

30

Pressure (bar)Fig. 7

Rapid Crack Propagation (S4 test) data: determined at Kiwa Gastec Certification

Regarding the processability, Fig. 8 shows the dynamic shear viscosity of New-PE100 at 190 degree
C compared with PE100-B. We improved the no-sag and the extrusion properties of New-PE100 by
giving it high shear sensitivity with very high viscosity at low frequency region.
1.E+07

New-PE100

1.E+06

η* (Pa･s)

PE100-B

1.E+05

1.E+04

1.E+03

1.E+02
1.E-03

1.E-02

1.E-01

1.E+00ω (rad/s)

Fig. 8

Dynamic shear viscosity data at 190 degree C

1.E+01

1.E+02

We evaluated extrusion properties, producing 110mm diameter SDR11 pipes by the temperature
pre-setting of 200 degree C.

We used 45mm screw diameter, 33 L/D Krauss-Maffei pipe molding

machine for the evaluation. Fig. 9 shows the relationship between screw speed and output.

Fig. 10shows the relationship between output and resin temperature. As shown in Fig. 9 and Fig. 10,
New-PE100 has nearly the same extrusion properties as those of PE100-B.
250

New-PE100
PE100-B

Output (kg/h)

200

150

100

50

0
0

100

200

300

400

Screw speed (rpm)Fig. 9Processability data-1: relationship between output and screw speed

300

Resin temperature (degree C)

New-PE100
PE100-B
250

200

150
0

50

100

150

200

Output (kg/h)

Fig. 10Processability data-2: relationship between resin temperature and output

250

Conclusion
The efforts to improve PE100 or to develop higher rating material like PE125 are continuing in many
PE manufactures to meet more demanding new installation methods and to contribute to enhancing
the safety margin of the PE pipe-line system. As previous studies and this study clarify, what is
important for excellent durability is to concentrate co-monomer in HMWC and distribute them as
evenly as possible. We could achieve such molecular architecture by precise and sophisticatedpolymerization control and develop new PE100 with excellent SCG resistance using our 3 cascadereactor process, AMSLP, and our high performance Ziegler-Natta catalyst. We believe that
continuous efforts of PE manufactures will further improve PE properties and will expand thepossibility of PE in the pipe industry.

References
1.

ASTM F 1473, Notch Tensile Test to Measure the Resistance to Slow Crack Growth ofPolyethylene Pipes and Resins

2.

ISO 16770, Plastics – Determination of environmental cracking (ESC) of polyethylene (PE) –Full-notch creep test (FNCT)3.

ISO13479, Polyolefin pipes for the conveyance of fluids – Determination of resistance to crack
propagation – Test method for slow crack growth on notched pipes4.

ISO 4437, Buried polyethylene (PE) pipes for the supply of gaseous fuels – Metric series Specifications

5.

ISO 4427, Polyethylene (PE) pipes for water supply – Specifications

6.

ISO 9080, Plastics piping and ducting systems – Determination of long-term hydrostatic
strength of thermoplastics materials in pipe form by extrapolation

7.

Y. L. Huang and N. Brown, J. Polym. Sci., 29, 129, 1991

8.

X. Lu, N. Ishikawa and N. Brown, J. Polym. Sci., 34, 1809, 1996

9.

E. T. Hsieh and J. C. Randal, Macromolecules, 15, 353, 1982

10.

E. T. Hsieh and J. C. Randal, Macromolecules, 15, 1402, 1982

11.

J. C. Randall, Macromolecules, 29, 201, 1989

12.

S. H. Beech and E. Q. Clutton, Plastics, Rubber and Composites, 34, 294, 2005

13.

ISO 14531-1, Plastics pipes and fittings - Crosslinked polyethylene (PE-X) pipe systems forthe conveyance of gaseous fuels - Metric series - Specifications -- Part 1: Pipes

14.

ISO 13477, Thermoplastics pipes for the conveyance of fluids - Determination of resistance torapid crack propagation (RCP) - Small-scale steady-state test (S4 test)